To someone who is getting sucked out to sea by a rip current, “don’t panic!” may be difficult to heed, even if that’s exactly what you should do. But lifeguards say to not only relax but flip over and float out of the danger.

Rip currents are one of the coast’s greatest dangers and account for the most beach rescues every year. About 100 people drown from rip currents along U.S. beaches each year, according to the United States Lifesaving Association. And more than 80 percent of beach rescues annually involve rip currents.

Already this year, there have been at least 21 people killed from rip currents in U.S. waters, according to the National Weather Service.

Rip currents are narrow columns of water flowing rapidly away from the beach. They don’t pull swimmers under water, but can carry them out a fair distance from shore.

“A rip current is like a river that pulls out to sea,” said San Diego Lifeguard Marine Safety Lieutenant Charlie Knight. “So when the waves come into the beach, it needs somewhere to go. And so it takes these little channels out that we call rip currents to put all that water back into the ocean.”

Low spots along the beach, or areas near jetties or piers, are often where rip currents form. They can be connected to stormy weather but also sometimes occur during sunny days. They can be hard to detect because the surface water often appears calm.

The current can flow as fast as eight feet per second (3.2 meters per second), faster than even a strong swimmer can overcome, according to the National Oceanic and Atmospheric Administration.

The most frequent advice from beach rescue teams and weather forecasters is to “’flip, float and follow.” Flipping over to float creates calm, conserves energy and keeps the airways open while the swimmer is in the rip current’s grip.

A relentless heatwave sweeping across Europe has turned rivers, lakes and beaches into deadly traps, with at least 40 people drowning in France since June 18 as millions sought relief from record-breaking temperatures. French Prime Minister Sébastien Lecornu said most of the victims were young people as the government convened an emergency meeting to respond to the escalating crisis. Temperatures climbed above 44C in parts of the country, forcing the Eiffel Tower and the Louvre to shorten opening hours, while rail services and a nuclear reactor were disrupted by the extreme heat. Similar conditions have spread across Italy, Spain, Germany and the UK, highlighting a continent-wide emergency as scientists warn the Sahara-driven heat dome could push temperatures even higher in the coming days.